MEASUREMENT OF THE SM HIGGS BOSON PROPERTIES IN THE DIPHOTON DECAY CHANNEL WITH THE ATLAS RUN 2 DATASET

On the 4th July 2012, the ATLAS and CMS collaborations announced the discovery of a particle with mass around 125 GeV compatible with the Higgs boson predicted by the Standard Model. The long sought Higgs boson was observed combining the three most sensitive decay channels in a hadron collider, the H->yy, H->4l and H->WW channels. The collaborations have then started a huge effort in assessing its properties with the highest possible precision and found them to agree with the ones predicted for the SM Higgs boson within the statistical precision. Nowadays, this program is not settled yet: the Higgs boson still attracts our interest because it interacts with almost all the Standard Model particles and it is considered a portal towards new physics beyond the SM. The work reported in this manuscript is fully part of this physic program, focusing on the measurement of the Higgs boson mass and production cross sections in the channel, performed with the pp collision dataset collected by ATLAS at LHC with a center-of-mass energy of 13 TeV and amounting to 139 fb^-1 . In this manuscript, the full analysis chain is described and particular emphasis is placed in the description of the innovative event categorization procedures I have­ designed for the coupling and mass measurements. The Higgs boson pro­­duction cross sections are measured in several different parametrizations in order to deeply explore its production phase space and represents the final Run 2 results for this decay channel. The results are up­­ to a factor 4 more precise than latest results and they have been found to be compatible with the Standard Model prediction. The expected result for the Higgs mass measurement is reported as well and it presents a sizeable improvement with respect to the latest published results.